Some metallic components of aircraft bodies exposed to higher temperature undesirably oxidize and become discolored. For example, aircraft utilizing titanium tailcones handling exhausts from auxiliary power units, are exposed to temperatures sufficient to oxidize the surface and turn it to a color typically ranging from a yellow straw or deep gold to blue or purple. This discoloration is not aesthetically pleasing and conventional paints lack the ability to withstand the temperatures experienced by the tailcone. In addition, manual handling of such components, such as manipulations or contact by service technicians or aircraft personnel and/or handling during manufacturing, frequently result in undesirable staining and/or discoloration, due to fingerprints or other surface contaminants.
Some known coating processes provide some corrosion protection; however, these processes typically use strong acids or strong bases and toxic materials (especially heavy metals such as chromates), these surface treatment processes are disadvantageous from an environmental viewpoint. For example, these processes require significant amounts of water to rinse excess process solutions from the treated parts. The rinse water and spent process solutions must be treated to remove dissolved metals prior to their discharge or reuse. Removing the metals generates additional hazardous wastes that are challenging to cleanup and dispose. Controlling exposure of workers to the hazardous process solutions during either tank or manual application requires special control and exposure monitoring equipment that increases the process cost. These measures greatly increase the cost of using these types of chemical processes.
One method utilized to provide oxidation resistance includes application of paint. Paints are commonly used to protect a surface and to provide color, gloss, reflectivity, or the like on a substrate. Paints generally disperse metal or ceramic pigments and a binder in a water or organic vehicle to form a film when dried on a surface. Typically, the binder is an organic resin. Therefore, paints are generally useable only at relatively low temperatures. They can be difficult to apply uniformly. They are relatively heavy and are expensive to repair. In addition, harsh environments, such as high temperatures, degrade typical organic resins.
What is needed is a coating and method of coating that provides protection against oxidation that renders metallic surfaces resistant to discoloration when exposed to high temperatures, such as the temperatures present in the exhaust of the auxiliary power unit of an aircraft, is less hazardous to the environment and is easily applied, requiring little excess heating and/or expensive processing equipment and/or techniques.